Among discharge tube operation devices used in liquid crystal backlights, etc., there are such devices that adjust the illuminance of the discharge tube by adjusting a current flowing through the discharge tube by feedback-controlling the current in the discharge tube, as disclosed in, for example, Unexamined Japanese Patent Application KOKAI Publication No. 2002-43088.
The general configuration of a conventional discharge tube operation device of this type is illustrated in FIG. 4. The conventional discharge tube operation device comprises a direct-current power source V3, a DC-AC(direct-current-alternating-current) conversion circuit 50, a resonance section 60, a discharge tube current detection circuit 70, a soft-start circuit 80, an error amplifier 83, a control circuit 87, a time division signal output circuit 85, and a reference voltage power source V4.
The DC-AC conversion circuit 50 converts a direct-current voltage supplied from the direct-current power source V3 to an alternating-current voltage by switching the voltage through MOSFETs 51 and 52.
The resonance section 60 comprises a transformer 61, a capacitor 62, and a discharge tube 63. The capacitor 62, a secondary coil 61b of the transformer 61, and the discharge tube 63 constitute a resonance circuit, which resonates at a unique resonance frequency.
The discharge tube current detection circuit 70 is constituted by diodes 71 and 72, and a resistor 73, detects the current level of a current I2 flowing through the discharge tube 63, and supplies an output signal to the soft-start circuit 80.
The soft-start circuit 80 is constituted by a resistor 81 and a capacitor 82, smoothes the output signal from the discharge tube current detection circuit 70, and supplies a signal E2 to a positive input terminal (+) of the error amplifier 83.
The error amplifier 83 is constituted by a differential amplifier, and a fixed reference voltage Vr from the reference voltage power source V4 is applied to a negative (inverting) input terminal (−) of the error amplifier 83. A capacitor 84 is connected between the output end of the error amplifier 83 and the output terminal of the reference voltage power source V4. The error amplifier 83 obtains the potential difference between the voltage of the signal E2 supplied from the soft-start circuit 80 and the reference voltage Vr, and supplies a voltage signal E3 to the control circuit 87.
The time division signal output circuit 85 has a luminance designation signal S3, which designates the luminance of the discharge tube 63, supplied to its input terminal. This luminance designation signal S3 indicates, for example, the ratio of a desired luminance to the rated luminance of the discharge tube 63. The time division signal output circuit 85 generates a time division signal S4 having a constant period and a variable duty ratio, in accordance with the designation by this luminance designation signal S3. That is, the time division signal output circuit 85 increases the ratio of a lit period (L-level period) occupied in one period in a case where the luminance designated by the luminance designation signal S3 is large, and reduces the ratio of a lit period (L-level period) occupied in one period in a case where the luminance designated by the luminance designation signal S3 is small.
The voltage of the time division signal S4 output by the time division signal output circuit 85 is added to the voltage of the output signal E2 from the soft-start circuit 80 and then supplied to the positive input terminal of the error amplifier 83. Accordingly, in a period in which the time division signal S4 is H level, an H level is applied to the positive input terminal of the error amplifier 83 regardless of the voltage level of the output signal E2 of the soft-start circuit 80, while in a period in which the time division signal S4 is L level, a voltage of almost the same level as the voltage level of the output signal E2 of the soft-start circuit 80 is applied to the positive input terminal of the error amplifier 83.
The control circuit 87 switches on or off the MOSFETs 51 and 52 in a manner that the voltage of the output signal E2 of the soft-start circuit 80 and the reference voltage Vr will be the same.
Next, the operation of the discharge tube operation device having the above-described configuration will be explained.
When given an instruction to light the discharge tube 63, the control circuit 87 starts the operation of switching on or off the MOSFETs 51 and 52. In response to this, a direct-current voltage is switched and an alternating-current voltage is output from the DC-AC conversion circuit 50. This alternating-current voltage is applied to a primary coil 61a of the transformer 61. A resonance voltage due to the resonance effect of the resonance section 60 is induced in the secondary coil 61b and applied to the discharge tube 63, thereby the discharge tube 63 is lit.
The discharge tube current detection circuit 70 detects the current level of a current 12 flowing through the discharge tube 63, and outputs a voltage corresponding to the detected current level from the cathode of a diode 71. The soft-start circuit 80 smoothes the output signal from the discharge tube current detection circuit 70, and supplies a signal E2 to the positive input terminal of the signal error amplifier 83.
The error amplifier 83 supplies a voltage signal E3 corresponding to the potential difference between the voltage of the signal E2 supplied from the soft-start circuit 80 and the reference voltage Vr to the control circuit 87. The control circuit 87 controls the switching frequencies of the MOSFETs 51 and 52 in a manner that the output signal E2 from the soft-start circuit 80 (=terminal voltage E2 of the capacitor 82) and the reference voltage Vr will have no potential difference.
With repetition of this control operation, the discharge tube current I2 is adjusted to a level corresponding to the reference voltage Vr.
After lighting the discharge tube 63, the discharge tube operation device adjusts the luminance of the discharge tube 63 to the luminance level designated by the designation signal S3 supplied to the time division signal output circuit 85. Hereinafter, the method of adjusting the luminance of the discharge tube 63 will be explained with reference to FIGS. 5.
FIG. 5A to FIG. 5D show the time division signal S4, the terminal voltage E2 of the capacitor 82, the voltage signal E3 of the error amplifier 83, and the current I2 of the discharge tube 63 respectively.
In FIG. 5, t0 and t5 indicate the timings at which the time division signal S4 supplied to the error amplifier 83 rises to H level, and t1 indicates the timing at which the time division signal S4 falls to L level.
The time division signal output circuit 85 determines the duty ratio of the time division signal S4 in accordance with the luminance level designated by the luminance designation signal S3, and outputs the time division signal S4 having the determined duty ratio.
When the time division signal S4 becomes H level at the timing t0 as shown in FIG. 5A, the voltage (=terminal voltage of the capacitor 82) E2 of the positive input terminal of the error amplifier 83 increases as shown in FIG. 5B. In response to this, the voltage signal E3 of the error amplifier 83 increases as shown in FIG. 5C.
The control circuit 87 controls the switching frequencies of the MOSFETs 51 and 52 such that they will differ from the resonance frequency, based on the increased voltage signal E3 of the error amplifier 83. At this time, no resonance voltage is generated because the resonance section 60 is not excited. Accordingly, the discharge tube current 12 is shut off as shown in FIG. 5D.
Next, when the time division signal S4 changes from H to L level at the timing t1, the voltage E2 of the output signal of the soft-start circuit 80 is applied, almost as is, to the positive input terminal of the error amplifier 83. This voltage E2 gradually decreases as shown in FIG. 5B, because the capacitor 82 gradually discharges.
After this, when the terminal voltage (=E2) of the capacitor 82 becomes closer to the reference voltage Vr at the timing t2, the voltage signal E3 of the error amplifier 83 decreases as shown in FIG. 5C.
The control circuit 87 controls the switching frequencies of the MOSFETs 51 and 52 such that they will approach the resonance frequency of the resonance section 60, based on the decreasing voltage signal E3 of the error amplifier 83. Due to this, the resonance section 60 is again excited to generate a resonance voltage. Accordingly, as shown in FIG. 5D, the discharge tube current I2 flows and the discharge tube 63 is lit (t=3).
After the discharge tube 63 is lit, the control circuit 87 performs feedback control in a manner that the potential difference between the terminal voltage E2 of the capacitor 82 and the reference voltage Vr will become extinct. Then, the current level of the current I2 of the discharge tube 63 is controlled.
In this manner, this discharge tube operation device adjusts the lit period and unlit period of the discharge tube 63 according to repetition of H level and L level of the time division signal S4.
In the conventional discharge tube operation device, if a time constant τ, which is determined by the resistance of the resistor 81 and capacitance of the capacitor 82 of the soft-start circuit 80, is small, an overrun is caused due to a delay in the feedback control system. Because of the overrun, a surge occurs in the current I2 flowing through the discharge tube 63 at the timing t3 in FIG. 5D. The occurrence of this surge will be a cause of shortening the life of the discharge tube 63.
To prevent the occurrence of a surge, the time constant τ of the soft-start circuit 80 may be set large. FIGS. 6A to D show the time division signal S4, terminal voltage E2 of the capacitor 82, voltage signal (output signal) E3 of the error amplifier 83, and current I2 of the discharge tube 63 of a case where the time constant is large.
When the time constant τ is large, a period (a period from t1 to t2) before the output voltage E3 of the error amplifier 83 starts decreasing becomes large in proportion to the time constant τ of the soft-start circuit 80, as shown in FIG. 6C.
That is, the time taken from the timing t1 at which the time division signal S4 becomes L level to the timing t3 at which the discharge tube current I2 starts flowing through the discharge tube 63 increases, as shown in FIG. 6D.
Due to this, a gap is produced between the period in which the time division signal S4 is L level and the period in which the discharge tube current I2 is flowing and the lit period t3 to t5 of the discharge tube is shortened, as shown in FIG. 6A and FIG. 6D. Since the lit period of the discharge tube 63 is short, the light-emitting luminance of the discharge tube 63 results in a level lower than designated by the luminance designation signal.
As described above, the discharge tube operation device having the conventional soft-start circuit 80 encounters the case where the luminance level of the discharge tube 63 does not reach the luminance level designated by the luminance designation signal S3, if the time constant τ of the soft-start circuit 80 is set large in order to suppress occurrence of a surge.